html, body {
  height: 100%;
  margin: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: #f5f5f5;
  font-family: sans-serif;
}

#app {
  width: 750px;
  height: 500px;
  background-color: #fff;
  box-shadow: 0 8px 24px rgba(0,0,0, .12);

  /*容器属性*/
  display: flex;
  flex-direction: column; /* 纵向排列 */
}

h1, p {
  margin: 0;
}